1. Field of the Invention
This invention relates to radioactivity well logging during drilling.
2. The Prior Art
It has long been the practice to measure radioactivity (either natural or induced) in formations penetrated by a well. Originally, to conduct such logging operations, drilling was stopped, the drill string removed from the well, and a radioactivity well logging tool run into the well on an electrically conductive cable to measure the radioactivity of formations of interest.
Many prior art patents disclose well logging of natural gamma ray intensity, or induced radioactivity (using either a gamma ray or a neutron source and detecting gamma rays scattered back to the logging instrument from the formation, or detecting neutrons slowed by the formation and scattered back to the logging instrument). For example, U.S. Pat. No. 2,481,014 to Herzog (1949) discloses natural gamma ray logging with a tool lowered into and raised from a well on an electrically conductive cable. U.S. Pat. No. 2,666,146 to Herzog (1950) discloses the use of a neutron source to bombard formations surrounding a well bore with neutrons to cause gamma rays to be emitted from the formation due to nuclear collision of the neutrons with atoms in the formation. U.S. Pat. No. 2,857,522 to Jones (1958) discloses well logging equipment adapted to be lowered into a well from which the drill string has been removed. The Jones well logging equipment discloses detecting beta rays with a scintillator mounted in the sidewall of a logging tool (sonde) adapted to be lowered into and removed from a well from which the drill string has previously been removed.
To avoid the cost of having to stop drilling, withdraw the drill string, enter the well with special well logging equipment, remove the well logging equipment, and re-enter the well bore with the drill string, systems have been developed for logging without removing the drill string. For example, U.S. Pat. No. 4,351,037 to Scherbatskoy (1982) and U.S. Pat. No. 4,216,536 to More (1980) refer to gamma ray well logging with the drill string in the hole, but they do not disclose how to mount the radioactivity sensors in the drill string.
Using radioactivity detectors mounted within the drill string so that natural gamma ray measurements can be made without having to remove the drill string from the well has the advantage of logging formations without first having to remove the drill string from the well. However, logging with such a system is of limited value because mounting the radioactivity sensor within the drill string significantly reduces sensitivity. For example, in natural gamma ray logging, the measured gamma ray intensity with a sensor inside the drill string (i.e., within the thick-walled drill collars at the lower end of the drill string) is only about 10% of that compared with conventional well logging tools where there is no drill collar to shield the natural radioactivity of the formation. The drill collar wall can attenuate neutron flux by a factor of 5. Moreover, mounting the radioactivity sensors within the drill collar makes it difficult to service and replace them in the field.